netdev - Re: [PATCH v2 net] net: solve a NAPI race

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Date:   Tue, 28 Feb 2017 17:04:13 -0800
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Francois Romieu <romieu@...zoreil.com>
Cc:     David Miller <davem@...emloft.net>, eric.dumazet@...il.com,
        netdev@...r.kernel.org, tariqt@...lanox.com, saeedm@...lanox.com
Subject: Re: [PATCH v2 net] net: solve a NAPI race

On Wed, 1 Mar 2017 01:22:40 +0100
Francois Romieu <romieu@...zoreil.com> wrote:

> David Miller <davem@...emloft.net> :
> > From: Eric Dumazet <eric.dumazet@...il.com>
> > Date: Mon, 27 Feb 2017 08:44:14 -0800
> >   
> > > Any point doing a napi_schedule() not from device hard irq handler
> > > is subject to the race for NIC using some kind of edge trigger
> > > interrupts.
> > > 
> > > Since we do not provide a ndo to disable device interrupts, the
> > > following can happen.  
> > 
> > Ok, now I understand.
> > 
> > I think even without considering the race you are trying to solve,
> > this situation is really dangerous.
> > 
> > I am sure that every ->poll() handler out there was written by an
> > author who completely assumed that if they are executing then the
> > device's interrupts for that NAPI instance are disabled.  And this is
> > with very few, if any, exceptions.  
> 
> Shareable pci irq used to remind author that such an assumption was
> not always right. Otoh it was still manageable as long as level only
> triggered irq were involved.
> 

When I had to deal with that in sky2, the best way was to have a single
NAPI poll handler shared between both ports. Works well and avoids races
in interrupt handling and enabling.